Film bulk acoustic resonators in thin ln-lt layers

ABSTRACT

Acoustic resonator devices, filter devices, and methods of fabrication. A resonator device includes a piezoelectric plate having a front surface and a back surface opposite the front surface, a back-side conductor pattern formed on the back surface, and a first front-side conductor pattern and a second front-side conductor pattern formed on respective portions of the front surface opposite the back-side conductor pattern. A portion of the piezoelectric plate between the first front-side conductor pattern and the back-side conductor pattern forms a first resonator and a portion of the piezoelectric plate between the second front-side conductor pattern and the back-side conductor pattern forms a second resonator.

RELATED APPLICATION INFORMATION

This patent is a continuation of patent application Ser. No. 17/125,779,entitled “FILM BULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Dec.17, 2020, which is a continuation of patent application Ser. No.17/090,599, entitled FILM BULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS,filed Nov. 5, 2020, now U.S. Pat. No. 10,994,380, which is acontinuation of patent application Ser. No. 16/932,719, entitled FILMBULK ACOUSTIC RESONATORS IN THIN LN-LT LAYERS, filed Jul. 18, 2020, nowU.S. Pat. No. 10,862,454, which claims priority from provisional patentapplication No. 62/875,855, entitled FILM ACOUSTIC RESONATORS IN THINLN-LT LAYERS, filed Jul. 18, 2019, and provisional application No.62/958,851, entitled YBAR ON ROTATED Y-CUTS OF LN, filed Jan. 9, 2020,the entire contents of each are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and schematic cross-sectionalviews of a one section (one period) of a Y-cut film bulk acousticresonator (YBAR).

FIG. 2 includes a schematic plan view and schematic cross-sectionalviews of a solidly-mounted Y-cut film bulk acoustic resonator (SM YBAR).

FIG. 3 is a graphical representation of Euler angles.

FIG. 4 is a chart of admittance as function of frequency of an exemplarySM YBAR.

FIG. 5 is a chart of piezo coefficients as a function of the secondEuler angle (3 for lithium niobate.

FIG. 6 is a chart of admittance as a function of frequency of anexemplary YBAR.

FIG. 7A is a graphic illustrating a non-rectangular electrode shape fora YBAR.

FIG. 7B is a graphic illustrating another non-rectangular electrodeshape for a YBAR.

FIG. 8 is a schematic circuit diagram and layout of a filter usingYBARs.

FIG. 9 is a flow chart of a process of fabricating a YBAR.

FIG. 10 is a flow chart of a process of fabricating a SM YBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

FIG. 1 shows a simplified top view and a cross-sectional view of oneperiod of a Y-cut film bulk acoustic resonator (YBAR) 100. The YBAR 100is made up of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithium niobate(LN), lithium tantalate (LT), lanthanum gallium silicate, or galliumnitride. The piezoelectric plate is cut such that the orientation of theX, Y, and Z crystalline axes with respect to the front and back surfacesis known and consistent.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The piezoelectric plate 110 may bebonded to the substrate 120 using a wafer bonding process, or grown onthe substrate 120, or attached to the substrate in some other manner.The piezoelectric plate may be attached directly to the substrate or maybe attached to the substrate via one or more intermediate materiallayers.

A cavity 125 is formed in the substrate 120 such that the portion of thepiezoelectric plate 110 containing front-side conductor patterns 130,132 is suspended over the cavity 125 without contacting the substrate120. “Cavity” has its conventional meaning of “an empty space within asolid body.” The cavity 125 may be a hole completely through thesubstrate 120 (as shown in Section A-A) or a recess in the substrate 120that does not extend through the substrate 120. The cavity 125 may beformed, for example, by selective etching of the substrate 120 before orafter the piezoelectric plate 110 and the substrate 120 are attached. Asshown in FIG. 1 , the cavity 125 has a rectangular shape. A cavity of aYBAR may have a different shape, such as a regular or irregular polygon.The cavity of a YBAR may more or fewer than four sides, which may bestraight or curved.

First front-side conductor pattern 130 and second front-side conductorpattern 132 are formed on the first surface 112 of the piezoelectricplate 110. A third back-side conductor pattern 134 is formed on thesecond surface 114 of the piezoelectric plate 110. The back-sideconductor pattern 134 can be a “floating” conductor pattern, meaningthat is not electrically connected to any other conductor. The conductorpatterns may be molybdenum, aluminum, copper, or some other conductivemetal or alloy. The portion of the piezoelectric plate 110 between thefirst front-side conductor pattern 130 and the back-side conductorpattern 134 forms a first resonator. The portion of the piezoelectricplate 110 between the second front-side conductor pattern 132 and theback-side conductor pattern 134 forms a second resonator. The first andsecond resonators are electrically in series such that an RF signalapplied between the first and second front-side conductor patterns 130,132 excites acoustic waves in both the first and second resonators.

The piezoelectric plate may be X-cut or Y-cut (i.e. with the X or Ycrystalline axis of the piezoelectric material normal to the surfaces112, 114). In this case, an RF signal applied between the first andsecond front-side conductor patterns 130, 132 will excite shear acousticwaves in both the first and second resonators. Rotated Y-cuts can beused to achieve shear displacements exclusively in the sagittal plane,and to control the electromechanical coupling of the resonators.

As shown in FIG. 1 , the first and second front-side conductor patterns130, 132 each consist of a single electrode, and the conductor patterns130, 132, 134 are rectangular in shape. The conductor patterns may benon-rectangular (e.g. trapezoidal), curved, or irregular to suppressparasitic acoustic modes.

Alternatively, the first and second front-side conductor patterns 130,132 may form an interleaved finger pattern (IFP) (not shown in FIG. 1 ,but similar to IFP 230 of FIG. 2 ). In this case, the first front-sideconductor pattern 130 includes a first plurality of parallel fingersextending from a first busbar. The second front-side conductor pattern132 includes a second plurality of parallel fingers extending from asecond busbar. The first and second pluralities of parallel fingers areinterleaved. The width m of each finger will be a substantial portion ofthe pitch p, or center-to-center spacing, of the fingers.

In the detailed cross-sectional view, the thickness of the piezoelectricplate 110 is dimension ts and the thickness of the conductor patterns130, 132, 134 is dimension tm. The thickness ts of the piezoelectricplate may be, for example, 100 nm to 1000 nm. The thickness tm of theconductor patterns 130, 132, 134 may be, for example, 10 nm to 500 nm.The thickness of the conductor patterns may be the same or one conductorpattern may have a different thickness from the others.

The piezoelectric plate 110 may be etched or otherwise removed,completely or only partially, in the area between the first and secondfront-side conductor patterns 130, 132, forming slots 115. The presenceof the slots 115 may suppress lateral acoustic modes that might beexcited by the electric field between the front-side conductor patterns130, 132. A depth tg of the slot 115 can extend partially or completelythrough the piezoelectric plate 110.

FIG. 2 shows a simplified schematic top view and a cross-sectional viewof a solidly-mounted Y-cut film bulk acoustic resonator (SM YBAR) 200.The SM YBAR 200 includes a piezoelectric plate 210 having parallel frontand back surfaces (shown in the cross-sectional view). The piezoelectricplate 210 is a thin single-crystal layer of a piezoelectric materialsuch as lithium niobate, lithium tantalate, lanthanum gallium silicate,gallium nitride, or aluminum nitride. The piezoelectric plate is cutsuch that the orientation of the X, Y, and Z crystalline axes withrespect to the front and back surfaces is known and consistent. Thepiezoelectric plate may be Z-cut, X-cut, Y-cut or rotated Y-cut aspreviously described.

First and second front-side conductor patterns 230, 232 are formed onthe front surface of the piezoelectric plate 210. The first and secondfront-side conductor patterns 230, 232 form an interleaved fingerpattern. The first front-side conductor pattern 230 includes a firstplurality of parallel fingers extending from a first busbar. The secondfront-side conductor pattern 232 includes a second plurality of parallelfingers extending from a second busbar. The first and second pluralitiesof parallel fingers are interleaved. The width m of each finger will bea substantial portion of the pitch p, or center-to-center spacing, ofthe fingers. The first and second front-side conductor patterns 230,232, are not necessarily an IFP, but may be single electrodes similar toconductor patterns 130, 132 shown in FIG. 1 .

The back surface of the piezoelectric plate 210 is attached to, andmechanically supported by, a substrate 220. The substrate 220 may be,for example, silicon, sapphire, quartz, or some other material. Unlikethe YBAR 100 of FIG. 1 , there is not a cavity in the substrate underthe conductor patterns. Instead, an acoustic Bragg reflector 240 issandwiched between the substrate 220 and the back surface of thepiezoelectric plate 210. The term “sandwiched” means the acoustic Braggreflector 240 is both disposed between and physically connected to asurface of the substrate 220 and the back surface of the piezoelectricplate 210. In some circumstances, thin layers of additional materialsmay be disposed between the acoustic Bragg reflector 240 and the surfaceof the substrate 220 and/or between the acoustic Bragg reflector 240 andthe back surface of the piezoelectric plate 210. Such additionalmaterial layers may be present, for example, to facilitate bonding thepiezoelectric plate 210, the acoustic Bragg reflector 240, and thesubstrate 220.

The acoustic Bragg reflector 240 includes multiple layers that alternatebetween materials having high acoustic impedance and materials havinglow acoustic impedance. “High” and “low” are relative terms. For eachlayer, the standard for comparison is the adjacent layers. Each “high”acoustic impedance layer has an acoustic impedance higher than that ofboth the adjacent low acoustic impedance layers. Each “low” acousticimpedance layer has an acoustic impedance lower than that of both theadjacent high acoustic impedance layers. Each of the layers has athickness equal to, or about, one-fourth of the acoustic wavelength ator near a resonance frequency of the SM XBAR 200. Materials havingcomparatively low acoustic impedance include silicon dioxide, siliconoxycarbide, aluminum, titanium, and certain plastics such ascross-linked polyphenylene polymers. Materials having comparatively highacoustic impedance include silicon nitride, aluminum nitride, siliconcarbide, and metals such as molybdenum, tungsten, gold, and platinum.All of the high acoustic impedance layers of the acoustic Braggreflector 240 are not necessarily the same material, and all of the lowacoustic impedance layers are not necessarily the same material.Preferably, the high acoustic impedance layer nearest the piezoelectricplate will be a conductive metal, which performs the function of thethird conductor pattern (134 in FIG. 1 ). In the example of FIG. 2 , theacoustic Bragg reflector 240 has a total of four layers. An acousticBragg reflector may have more than, or less than, four layers.

The piezoelectric plate may be etched or otherwise removed, completelyor only partially, in the area between the first and second front-sideconductor patterns 230, 232, forming slots 215. The presence of theslots 215 may suppress lateral acoustic modes that might be excited bythe electric field between the front-side conductor patterns 230, 232. Aslot may also be formed between the first and second front-sideconductor patterns 130, 132 of the YBAR 100 if the back-side conductorpattern 134 is sufficiently thick to provide mechanical support to thepiezoelectric plate 110.

FIG. 3 is a graphical illustration of Euler angles 300. Euler angles area system, introduced by Swiss mathematician Leonhard Euler, to definethe orientation of a body with respect to a fixed coordinate system. Theorientation is defined by three successive rotations about angles α, β,and γ.

As applied to acoustic wave devices, xyz is a three-dimensionalcoordinate system aligned with the crystalline axes of the piezoelectricmaterial. XYZ is a three-dimensional coordinate system aligned with theacoustic wave device, where the Z axis is normal to the surface of thepiezoelectric material and XY is the plane of the surface of thepiezoelectric material. The vector N is the intersection of the XY andxy planes. The vector N is also the common perpendicular to the z and Zaxis.

FIG. 4 is a chart 400 of absolute value of admittance as function offrequency of an SM YBAR similar to the SM YBAR of FIG. 2 with anacoustic Bragg reflector and slots similar to slots 215. In thisexample, the piezoelectric plate is formed of lithium niobate and theEuler angles are (0, 90°, 0) such that the Y-crystalline axis is normalto the piezoelectric plate and the Z-crystalline axis is parallel toslots between the front-side conductor patterns. tm is 100 nm, ts is 400nm, p is 5 um, and m is 4.5 um. In this periodic simulation, theresonance frequency Fr is 3389 MHz, as shown in curve 410. Q is 416. Inthis example, spurious modes have been reduced due to the presence ofgaps in LN preventing excitation of horizontally propagating waves.

In other YBARs, rotated Y-cut LN piezoelectric plates provide tunablecoupling through adjustment of the angle of rotation, particularly forpiezoelectric plates with slots corresponding to the conductor patterns.Different crystal orientations of the piezoelectric plates withdifferent couplings can be used to adapt the YBAR to the differentfilter specifications. Using a rotated Y-cut LN piezoelectric plateallows only one shear mode to be excited, which reduces parasitic modescompared to other crystal cuts.

FIG. 5 is a chart of piezo coefficients as a function of β of anexemplary YBAR. Curve 510 shows piezo coefficients for the piezoconstant e34, demonstrating coupling to the vertical electric fielddirected perpendicular to the piezoelectric plate along the Z-axis forangles of β. Curve 510 shows that coupling to shear wave is maximal fora rotated Y-cut piezoelectric plate that has a rotation angle β of 81.5degrees. Curve 520 shows the piezo coefficients for the piezo constante32, with e32 close to zero for β=81.5 degrees. This indicates thatthere are minimal or no undesirable compression waves in they-direction. Rotation angle β is not restricted to any particular angleand can be tuned for particular applications to an angle that willprovide desirable results. In one example, rotation angle β can be in arange from 60 degrees to 100 degrees or 70 degrees to 90 degrees.

FIG. 6 is a chart of absolute value of admittance as a function offrequency for an exemplary YBAR with non-rectangular electrodes arotated Y-cut piezoelectric plate that has a rotation angle β of 81.5degrees and a floating back-side conductor pattern. In this example, theconductor patterns are formed of Aluminum, p is 10 um, m is 8 um, tm is50 nm, ts is 400 nm. Curve 610 shows resonance frequency is 3937.796MHz, resonance Q is 560, resonance admittance is 1.294, antiresonancefrequency is 4928.785 MHz, antiresonance Q is 600, and resonance toantiresonance ratio is 22.3% (991 MHz). In this example, resonance toantiresonance ratio is improved and spurious modes are reduced due tothe presence of gaps in LN preventing excitation of horizontallypropagating waves.

FIG. 7A is a graphic illustrating a non-rectangular electrode shape fora YBAR. In FIG. 7A, the conductor pattern 700 includes a firsttrapezoidal electrode shape and a corresponding second trapezoidalelectrode shape, such that the first and second shapes can beinterleaved with a constant distance between edges. FIG. 7B is a graphicillustrating another non-rectangular electrode shape for a YBAR. In FIG.7B, the conductor pattern 750 includes a first parallelogram electrodeshape and a corresponding second parallelogram electrode shape, suchthat the first and second shapes can be interleaved with a constantdistance between edges. The non-rectangular electrode shapes of FIGS. 7Aand 7B reduce or prevent undesirable wave reflections resulting inparasitic modes. The non-rectangular shapes are not limited to thoseshown in FIGS. 7A and 7B. The non-rectangular shapes can be morecomplicated shapes, such as curved fish-shapes or differentinclinations.

FIG. 8 is a schematic circuit diagram for a high frequency band-passfilter 800 using YBARs. The filter 800 has a conventional ladder filterarchitecture including three series resonators 810A, 810B, 810C and twoshunt resonators 820A, 820B. The three series resonators 810A, 810B, and810C are connected in series between a first port and a second port. InFIG. 8 , the first and second ports are labeled “In” and “Out”,respectively. However, the filter 800 is bidirectional and either portcan serve as the input or output of the filter. The two shunt resonators820A, 820B are connected from nodes between the series resonators toground. All the shunt resonators and series resonators are YBARs.

The filter 800 may include a substrate having a surface and asingle-crystal piezoelectric plate having parallel front and backsurfaces, with a back-side conductor pattern coupled to the back surfacefor each YBAR. Back-side conductor patterns of the YBARs areelectrically isolated from each other. The back-side conductor patterncan be like back-side conductor 134 of FIG. 1 or can be an acousticBragg reflector sandwiched between the surface of the substrate and theback surface of the single-crystal piezoelectric plate like acousticBragg reflector 240 of FIG. 2 . In the example shown in FIG. 8 , thesubstrate, acoustic Bragg reflector, and single-crystal plates arerepresented by the rectangle 810 in FIG. 8 . A front-side conductorpattern formed on the front surface of the single-crystal piezoelectricplate includes a conductor pattern for each of the three seriesresonators 810A, 810B, 810C and two shunt resonators 820A, 820B. Theconductor pattern can be similar to the front-side conductor patterns130, 132 of FIG. 1 , or the interleaved finger pattern (IFPs) 230, 232of FIG. 2 . All of the conductor patterns are configured to excite shearacoustic waves in the single-crystal piezoelectric plate in response torespective radio frequency signals applied to each conductor pattern.

In a ladder filter, such as the filter 800, the resonance frequencies ofshunt resonators are typically lower than the resonance frequencies ofseries resonators. The resonance frequency of a YBAR is determinedprimarily by the thickness of the piezoelectric plate and conductorlayers. The resonance frequency of a YBAR resonator also has a weakdependence on conductor pattern pitch. Conductor pattern pitch alsoimpacts other filter parameters including impedance and power handlingcapability.

To reduce the resonance frequencies of some or all of the shuntresonators relative to the series resonators, a first dielectric layer(represented by the dashed rectangle 825) having a first thickness t1may be deposited over the front-side conductor patterns of one or bothof the shunt resonators 820A, 820B. A second dielectric layer(represented by the dashed rectangle 815) having a second thickness t2,less than t1 may be deposited over the front-side conductor patterns ofthe series resonators 810A, 810B, 810C. The thickness of each of thefirst and second dielectric layers may be between 0 and 300 nm, suchthat 0<t2<t1<300 nm. The use of two different dielectric layerthicknesses may be appropriate in situations where a shift of at least100 MHz is required between the resonance frequencies of series andshunt resonators. When the dielectric layers are silicon dioxide,t1−t2>25 nm is sufficient to cause a shift of at least 100 MHz betweenthe resonance frequencies of series and shunt resonators.

DESCRIPTION OF METHODS

FIG. 9 is a simplified flow chart of a method 900 for making a YBAR or afilter incorporating YBARs, such as the YBAR of FIG. 1 . The method 900starts at 910 with a piezoelectric plate disposed on a sacrificialsubstrate 902 and a device substrate 904. The method 900 ends at 995with a completed YBAR or filter. The flow chart of FIG. 9 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 9 .

Thin plates of single-crystal piezoelectric materials bonded to anon-piezoelectric substrate are commercially available. At the time ofthis application, both lithium niobate and lithium tantalate plates areavailable bonded to various substrates including silicon, quartz, andfused silica. Thin plates of other piezoelectric materials may beavailable now or in the future. The thickness of the piezoelectric platemay be between 300 nm and 1000 nm. When the substrate is silicon, alayer of SiO2 may be disposed between the piezoelectric plate and thesubstrate. The piezoelectric plate 902 may be, for example, z-cutlithium niobate with a thickness of 400 nm (as used in all of theprevious examples) bonded to a silicon wafer with an intervening SiO2layer. The device substrate 904 may be silicon (as used in the previousexamples) fused silica, quartz, or some other material.

At 920, the piezoelectric plate on the sacrificial substrate 902 and thedevice substrate 904 are bonded. In YBARs with a floating back-sideconductor pattern as shown in FIG. 1 , cavities in the device substrate904 can be positioned to correspond with the back-side conductorpattern. The piezoelectric plate on the sacrificial substrate 902 andthe device substrate 904 may be bonded using a wafer bonding processsuch as direct bonding, surface-activated or plasma-activated bonding,electrostatic bonding, or some other bonding technique.

After the piezoelectric plate on the sacrificial substrate 902 and thedevice substrate 904 are bonded, the sacrificial substrate, and anyintervening layers, are removed at 930 to expose the surface of thepiezoelectric plate (the surface that previously faced the sacrificialsubstrate). The sacrificial substrate may be removed, for example, bymaterial-dependent wet or dry etching or some other process.

A front-side conductor pattern, such as front-side conductor pattern130, 132 of FIG. 1 , is formed at 940 by depositing and patterning oneor more conductor layers on the surface of the piezoelectric plate thatwas exposed when the sacrificial substrate was removed at 930. Theconductor pattern may be, for example, aluminum, an aluminum alloy,copper, molybdenum a copper alloy, or some other conductive metal.Optionally, one or more layers of other materials may be disposed below(i.e. between the conductor layer and the piezoelectric plate) and/or ontop of the conductor layer. For example, a thin film of titanium,chrome, or other metal may be used to improve the adhesion between theconductor layer and the piezoelectric plate. A conduction enhancementlayer of gold, aluminum, copper or other higher conductivity metal maybe formed over portions of the front-side conductor pattern (for examplethe IFP bus bars and interconnections between the IFPs).

The front-side conductor pattern may be formed at 940 by depositing theconductor layer and, optionally, one or more other metal layers insequence over the surface of the piezoelectric plate. The excess metalmay then be removed by etching through patterned photoresist. Theconductor layer can be etched, for example, by plasma etching, reactiveion etching, wet chemical etching, and other etching techniques.Further, portions of the piezoelectric plate between the conductors ofthe front-side conductor pattern can be removed to form grooves in thepiezoelectric plate between the conductors. For example, the portionscan be removed during the same or a different etching process. Theportions may be removed through an entire thickness of the piezoelectricplate or only to a certain depth.

Alternatively, the front-side conductor pattern may be formed at 940using a lift-off process. Photoresist may be deposited over thepiezoelectric plate. and patterned to define the front-side conductorpattern. The conductor layer and, optionally, one or more other layersmay be deposited in sequence over the surface of the piezoelectricplate. The photoresist may then be removed, which removes the excessmaterial, leaving the conductor pattern.

At 950, one or more optional front-side dielectric layers may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. The one or more dielectric layers maybe deposited using a conventional deposition technique such assputtering, evaporation, or chemical vapor deposition. The one or moredielectric layers may be deposited over the entire surface of thepiezoelectric plate, including on top of the front-side conductorpattern. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the front-side conductor patterns. Masks may alsobe used to allow deposition of different thicknesses of dielectricmaterials on different portions of the piezoelectric plate. For example,a first dielectric layer having a first thickness t1 may be depositedover the front-side conductor patterns of one or more shunt resonators.A second dielectric layer having a second thickness t2, where t2 isequal to or greater than zero and less than t1, may be deposited overthe conductor patterns of series resonators.

At 960, a back-side cavity is formed in the back side of thepiezoelectric plate, where the cavity corresponds to the position of thefront conductor pattern. The cavity may be formed using an anisotropicor orientation-dependent dry or wet etch to open a hole through theback-side of the substrate to the piezoelectric plate.

At 970, a back-side conductor pattern is formed on the back side of thepiezoelectric plate. The back-side conductor pattern can be formed to belike back-side conductor pattern 134 of FIG. 1 . The back-side conductorpattern can be formed by depositing and patterning one or more conductorlayers on the back surface of the piezoelectric plate. The back-sideconductor pattern may be, for example, aluminum, an aluminum alloy,copper, molybdenum a copper alloy, or some other conductive metal.Optionally, one or more layers of other materials may be disposed below(i.e. between the conductor layer and the piezoelectric plate) and/or ontop of the conductor layer. For example, a thin film of titanium,chrome, or other metal may be used to improve the adhesion between theconductor layer and the piezoelectric plate.

The back-side conductor pattern may be formed by depositing theconductor layer and, optionally, one or more other metal layers insequence over the surface of the piezoelectric plate. The excess metalmay then be removed by etching through patterned photoresist. Theconductor layer can be etched, for example, by plasma etching, reactiveion etching, wet chemical etching, and other etching techniques.

Alternatively, the back-side conductor pattern may be formed using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the back-side conductor pattern. Theconductor layer and, optionally, one or more other layers may bedeposited in sequence over the surface of the piezoelectric plate. Thephotoresist may then be removed, which removes the excess material,leaving the back-side conductor pattern.

After the back-side conductor pattern is formed at 970, the filterdevice may be completed at 980. Actions that may occur at 980 includingdepositing and patterning additional metal layers to form conductorsother than the conductor patterns; depositing anencapsulation/passivation layer such as SiO2 or Si3O4 over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 980is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at995.

A variation of the process 900 starts with a single-crystalpiezoelectric wafer at 902 instead of a thin piezoelectric plate on asacrificial substrate of a different material. Ions are implanted to acontrolled depth beneath a surface of the piezoelectric wafer (not shownin FIG. 9 ). The portion of the wafer from the surface to the depth ofthe ion implantation is (or will become) the thin piezoelectric plateand the balance of the wafer is the sacrificial substrate. The back-sideconductor pattern is formed, and the piezoelectric wafer and devicesubstrate are bonded at 920. At 930, the piezoelectric wafer may besplit at the plane of the implanted ions (for example, using thermalshock), leaving a thin plate of piezoelectric material exposed andbonded to the back-side conductor pattern. The thickness of the thinplate piezoelectric material is determined by the energy (and thusdepth) of the implanted ions. The process of ion implantation andsubsequent separation of a thin plate is commonly referred to as “ionslicing”.

FIG. 10 is a simplified flow chart of a method 1000 for making a SM YBARsimilar to that of FIG. 2 . The method 1000 starts at 1010 with apiezoelectric plate disposed on a sacrificial substrate 1002 and adevice substrate 1004. The method 1000 ends at 1095 with a completedYBAR or filter. The flow chart of FIG. 10 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, baking, annealing, monitoring, testing, etc.) maybe performed before, between, after, and during the steps shown in FIG.10 .

Lithium niobate and lithium tantalate plates can be bonded to varioussubstrates including silicon, quartz, and fused silica, where thethickness of the piezoelectric plate may be between 300 nm and 1000 nm.When the substrate is silicon, a layer of SiO2 may be disposed betweenthe piezoelectric plate and the substrate. The piezoelectric plate 1002may be, for example, z-cut lithium niobate with a thickness of 400 nm(as used in all of the previous examples) bonded to a silicon wafer withan intervening SiO2 layer. The device substrate 1004 may be silicon (asused in the previous examples) fused silica, quartz, or some othermaterial.

At 1020, an acoustic Bragg reflector is formed by depositing alternatinglayers of high acoustic impedance and low acoustic impedance materials.One or more layers can be deposited either or both the piezoelectricplate and the device substrate. Each of the layers has a thickness equalto or about one-fourth of the acoustic wavelength. Materials havingcomparatively low acoustic impedance include silicon dioxide, siliconoxycarbide, aluminum, and certain plastics such as cross-linkedpolyphenylene polymers. Materials having comparatively high acousticimpedance include silicon nitride, aluminum nitride, and metals such asmolybdenum, tungsten, gold, and platinum. All of the high acousticimpedance layers are not necessarily the same material, and all of thelow acoustic impedance layers are not necessarily the same material. Thetotal number of layers in the acoustic Bragg reflector may be from aboutfive to more than twenty. In filters including multiple SM YBARs, theacoustic Bragg reflector can then be etched to electrically isolate eachSM YBAR from the other SM YBARs.

At 1030, the piezoelectric plate on the sacrificial substrate 1002 andthe device substrate 1004 are assembled. In YBARs with acoustic Braggfilter as shown in FIG. 2 , the sacrificial substrate 1002 and thedevice substrate 1004 may be bonded such that the layers of the acousticBragg reflector are sandwiched between the piezoelectric plate and thedevice substrate. The piezoelectric plate on the sacrificial substrate1002 and the device substrate 1004 may be bonded using a wafer bondingprocess such as direct bonding, surface-activated or plasma-activatedbonding, electrostatic bonding, or some other bonding technique. Notethat, when one or more layers of the acoustic Bragg reflector aredeposited on both the piezoelectric plate and the device substrate, thebonding will occur between or within layers of the acoustic Braggreflector.

After the piezoelectric plate on the sacrificial substrate 1002 and thedevice substrate 1004 are bonded, the sacrificial substrate, and anyintervening layers, are removed at 1040 to expose the surface of thepiezoelectric plate (the surface that previously faced the sacrificialsubstrate). The sacrificial substrate may be removed, for example, bymaterial-dependent wet or dry etching or some other process.

A front-side conductor pattern, such as front-side conductor pattern230, 232 of FIG. 2 including IFPs, is formed at 1050 by depositing andpatterning one or more conductor layers on the surface of thepiezoelectric plate that was exposed when the sacrificial substrate wasremoved at 1040. The conductor pattern may be, for example, aluminum, analuminum alloy, copper, molybdenum a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the front-sideconductor pattern (for example the IFP bus bars and interconnectionsbetween the IFPs).

The front-side conductor pattern may be formed at 1050 by depositing theconductor layer and, optionally, one or more other metal layers insequence over the surface of the piezoelectric plate. The excess metalmay then be removed by etching through patterned photoresist. Theconductor layer can be etched, for example, by plasma etching, reactiveion etching, wet chemical etching, and other etching techniques.Further, portions of the piezoelectric plate between the conductors ofthe front-side conductor pattern can be removed to form grooves in thepiezoelectric plate between the conductors. For example, the portionscan be removed during the same or a different etching process. Theportions may be removed through an entire thickness of the piezoelectricplate or only to a certain depth.

Alternatively, the front-side conductor pattern may be formed at 1050using a lift-off process. Photoresist may be deposited over thepiezoelectric plate. and patterned to define the front-side conductorpattern. The conductor layer and, optionally, one or more other layersmay be deposited in sequence over the surface of the piezoelectricplate. The photoresist may then be removed, which removes the excessmaterial, leaving the conductor pattern.

At 1060, one or more optional front-side dielectric layers may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. The one or more dielectric layers maybe deposited using a conventional deposition technique such assputtering, evaporation, or chemical vapor deposition. The one or moredielectric layers may be deposited over the entire surface of thepiezoelectric plate, including on top of the front-side conductorpattern. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the front-side conductor patterns. Masks may alsobe used to allow deposition of different thicknesses of dielectricmaterials on different portions of the piezoelectric plate. For example,a first dielectric layer having a first thickness t1 may be depositedover the front-side conductor patterns of one or more shunt resonators.A second dielectric layer having a second thickness t2, where t2 isequal to or greater than zero and less than t1, may be deposited overthe conductor patterns of series resonators.

At 1070, in filters including multiple SM YBARs, the acoustic Braggreflector can be etched to electrically isolate the back-side conductorof each SM YBAR from the other SM YBARs. Portions of the acoustic Braggreflector may be removed by etching through a patterned photoresist, forexample, by plasma etching, reactive ion etching, wet chemical etching,and other etching techniques.

The filter device may then be completed at 1080. Actions that may occurat 1080 including depositing and patterning additional metal layers toform conductors other than the conductor patterns; depositing anencapsulation/passivation layer such as SiO2 or Si3O4 over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1080 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1095.

A variation of the process 1000 starts with a single-crystalpiezoelectric wafer at 1002 instead of a thin piezoelectric plate on asacrificial substrate of a different material. Ions are implanted to acontrolled depth beneath a surface of the piezoelectric wafer (not shownin FIG. 10 ). The portion of the wafer from the surface to the depth ofthe ion implantation is (or will become) the thin piezoelectric plateand the balance of the wafer is the sacrificial substrate. The back-sideconductor pattern is formed, and the piezoelectric wafer and devicesubstrate are bonded at 1020. At 1030, the piezoelectric wafer may besplit at the plane of the implanted ions (for example, using thermalshock), leaving a thin plate of piezoelectric material exposed andbonded to the back-side conductor pattern. The thickness of the thinplate piezoelectric material is determined by the energy (and thusdepth) of the implanted ions. The process of ion implantation andsubsequent separation of a thin plate is commonly referred to as “ionslicing”

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: apiezoelectric plate having a front surface and a back surface oppositethe front surface; a back-side conductor pattern formed on the backsurface; and a first front-side conductor pattern and a secondfront-side conductor pattern formed on respective portions of the frontsurface opposite the back-side conductor pattern, wherein a portion ofthe piezoelectric plate between the first front-side conductor patternand the back-side conductor pattern forms a first resonator and aportion of the piezoelectric plate between the second front-sideconductor pattern and the back-side conductor pattern forms a secondresonator.
 2. The device of claim 1, wherein the first and secondfront-side conductor patterns are electrically in series.
 3. The deviceof claim 1, wherein a radio frequency signal applied between the firstand second front-side conductor patterns excites respective acousticwaves in the respective first and second resonators.
 4. The device ofclaim 1, wherein the piezoelectric plate comprises X-cut, Y-cut orrotated Y-cut lithium niobate.
 5. The device of claim 1, wherein theback-side conductor pattern is floating.
 6. The device of claim 1,wherein the first conductor pattern consists of a single electrode andthe second conductor pattern consists of another single electrode. 7.The device of claim 1, wherein a portion of the piezoelectric platebetween the first and second conductor patterns is recessed to form aslot.
 8. A filter device comprising: a piezoelectric plate having afront surface and a back surface opposite the front surface; a pluralityof back-side conductor patterns formed on the back surface; and aplurality of first front-side conductor patterns and a plurality ofsecond front-side conductor patterns formed on respective portions ofthe front surface, wherein both one of the plurality of first front-sideconductor patterns and one of the plurality of second front-sideconductor patterns are opposite one of the plurality of back-sideconductor patterns, wherein a portion of the piezoelectric plate betweenthe one of the plurality of first front-side conductor patterns and theone of the plurality of back-side conductor patterns forms a firstresonator and a portion of the piezoelectric plate between the one ofthe plurality of second front-side conductor patterns and the one of theplurality of back-side conductor patterns forms a second resonator. 9.The device of claim 8, wherein the one of the plurality of firstfront-side conductor patterns and the one of the second plurality offront-side conductor patterns are electrically in series.
 10. The deviceof claim 8, wherein a radio frequency signal applied between the one ofthe plurality of first front-side conductor patterns and the one of thesecond plurality of front-side conductor patterns excites respectiveacoustic waves in the respective first and second resonators.
 11. Thedevice of claim 8, wherein the piezoelectric plate comprises X-cut,Y-cut or rotated Y-cut lithium niobate.
 12. The device of claim 8,wherein the one of the plurality of back-side conductor patterns isfloating.
 13. The device of claim 8, wherein the one of the plurality offirst conductor patterns consists of a single electrode and the one ofthe plurality of second conductor patterns consists of another singleelectrode.
 14. The device of claim 8, wherein a portion of thepiezoelectric plate between the one of the plurality of first conductorpatterns and the one of the plurality of second conductor patterns isrecessed to form a slot.
 15. A method of fabricating an acousticresonator device comprising: forming a floating back-side conductorpattern on a back surface of a piezoelectric plate, the piezoelectricplate having a front surface opposite the back surface; and forming afirst front-side conductor pattern and a second front-side conductorpattern on respective portions of the front surface opposite theback-side conductor pattern, wherein a portion of the piezoelectricplate between the first front-side conductor pattern and the back-sideconductor pattern forms a first resonator and a portion of thepiezoelectric plate between the second front-side conductor pattern andthe back-side conductor pattern forms a second resonator.
 16. The methodof claim 15, wherein the first and second front-side conductor patternsare electrically in series.
 17. The method of claim 15, wherein a radiofrequency signal applied between the first and second front-sideconductor patterns excites respective acoustic waves in the respectivefirst and second resonators.
 18. The method of claim 15, wherein thepiezoelectric plate comprises X-cut, Y-cut or rotated Y-cut lithiumniobate.
 19. The method of claim 15, wherein the back-side conductorpattern is floating.
 20. The method of claim 15, wherein a portion ofthe piezoelectric plate between the first and second conductor patternsis recessed to form a slot.